Patent Document 1 discloses a receiver that carries out signal demultiplexing according to a conventional QRM-MLD method (maximum likelihood detection (MLD) method using QR decomposition and M algorithm). As shown in FIG. 1, the receiver disclosed in Patent Document 1 has: a plurality of receiving antennas 10-1, 10-2, 10-3 and 10-4; channel estimation section 20; ranking section 30; rearranging section 40; QR decomposition section 50; signal converting section 60; maximum likelihood deciding section 70; and likelihood outputting section 80. Maximum likelihood deciding section 70 has four deciding sections 72-1, 72-2, 72-3 and 72-4. The number of deciding sections is determined according to the number of transmission signals. The deciding sections have similar processing blocks, and so fourth deciding section 72-4 will be described as a representative of these deciding sections. The deciding section has symbol replica generating section 74-4, square Euclidean distance calculating section 76-4 and surviving symbol candidate selecting section 78-4. Here, assume that signals x=(x1 . . . x4)T are each transmitted from four transmitting antennas by 16 QAM modulation scheme where the superscript letter symbol T stands for the transpose. Signals x are referred to as “transmission signal vectors” and form one symbol. x1, x2, x3 and x4 are referred to as “transmission signals” or “vector components.”
Channel estimation section 20 finds a channel impulse response value (CIR) or channel estimation value based on received signals including the pilot signal known both on the transmitting side and receiving side. Matrix H using channel estimation value hnm as a matrix element, is referred to as a “channel matrix.” Note that hnm represents the channel estimation value between the m-th transmitting antenna and n-th receiving antenna.
Ranking section 30 rates or ranks a plurality of received signals y1 . . . y4 in order of the magnitude of power.
Rearranging section 40 reports the order a plurality of received signals are arranged, to QR decomposition section 50 and signal converting section 60.
QR decomposition section 50 finds matrices Q and R such that channel matrix H determined in channel estimation section 20 is represented as a product of unitary matrix Q and upper triangular matrix R (H=QR). Unitary matrix Q in this case satisfies QHQ=QQH=I, and may be a square matrix or include different numbers of rows and columns. The superscript letter H represents the conjugate transpose and I represents the unit matrix.
Signal converting section 60 carries out signal conversion by multiplying received signal vectors y=(y1, . . . , y4)T by conjugate transpose matrix QH of unitary matrix Q. y=Hx=QRx holds between transmission signals and received signals. When QH is multiplied upon this equation from the left, QHy=z holds in the left side and QHQRx=Rx holds in the right side, so that the relationship between transmission signals and received signals can be represented by, for example, z=Rx. However, z=(z1 . . . z4)T=QHy holds. z is referred to as “received signal vectors after unitary conversion.”
Elements of received vector z can be represented as z1=r11x1+r12x2+r13x3+r14x4, z2=r22x2+r23x3+r24x4, z3=r33x3+r34x4 and z4=r44x4.
Maximum likelihood deciding section 70 narrows down candidates for a transmission signal (also referred to as “symbol candidates”), that is, decreases the number of candidates, by the maximum likelihood deciding method (MLD method). Symbol replica generating section 74-4 of deciding section 72-4 generates candidates of a transmission signal associated with received signal y4, using the matrix elements of upper triangular matrix R. The number of candidates is c, for example, and is set fixedly.
Square Euclidean distance calculating section 76-4 calculates square Euclidean distances between converted received signal z4 and C signal point candidates. The square Euclidean distances represent the metric used as the base of likelihood calculation. Candidates of shorter square Euclidean distances are decided to be closer to the transmitted symbol.
Surviving symbol candidate selecting section 78-4 outputs S1(≦C) candidates as surviving candidates based on the square Euclidean distances with respect to the candidates.
Likelihood outputting section 80 calculates the likelihoods or reliabilities of the candidates outputted from the surviving symbol candidate selecting section in the final stage. To be more specific, these likelihoods are represented by log likelihood ratios (LLR's). Outputs from likelihood outputting section 80 represent signal demultiplexing results and are transmitted to a subsequent demodulating section (for example, turbo decoder).
The operation will be described next. The receiver receives transmission signals as received signals y1 to y4 at four receiving antennas. These received signals are delivered to channel estimation section 20 and signal converting section 60. The order a plurality of received signals are arranged, is determined by channel estimation section 20, ranking section 30 and rearranging section 40. Here, the received signals are aligned in order of the magnitude of received power and, for ease of description, assume that received power increases in order from x1, x2, x3 and x4. Signal converting section 60 carries out unitary conversion of the received signals as in z=(z1 . . . z4)T=QHy and inputs the converted signals to maximum likelihood deciding section 70.
In the first stage in maximum likelihood deciding section 70, processing corresponding to default setting is carried out in deciding section 72-4. In this stage, the equation related to above z4 is focused upon. Matrix element r44 is known, and z4 does not interfere with other signals and relies on only one transmission signal x4. In this way, there are sixteen patterns of signal point candidates of transmission signal x4 at maximum. Symbol candidate generating section 74-4 generates sixteen signal point candidates (C=16) of x4. In other words, sixteen signal points on the signal constellation are selected. The square Euclidean distances between these candidates and converted fourth received signal z4 are calculated in square Euclidean distance calculating section 76-4, and S1 candidates are selected as surviving candidates in order from the shortest distance.
The second stage is performed in deciding section 72-3. Here, the equation related to z3 is focused upon. Matrix elements r33 and r34 are known, and there are sixteen patterns of signal candidates of x4 and sixteen patterns of signal candidates of x3. Sixteen signal points are introduced by symbol generating section 74-3 as additional signal points for x3. Consequently, there may be 16×16=256 patterns of combinations of signal points (that is, 256 candidates). The 256 patterns of square Euclidean distances between these candidates and third received signal x3 are calculated, and the candidates are narrowed down by selecting sixteen (S2=16) combinations in order from the smallest value.
Similar processing is carried out by deciding section 72-2 for the third stage. In this stage, the equation related to z2 is focused upon. Matrix elements r22, r23 and r24 are known and combinations of transmission signals x3 and x4 are narrowed down to sixteen patterns of candidates in the previous stage, and there are sixteen patterns of signal point candidates of x2. Consequently, symbol candidate generating section 74-2 generates sixteen candidates of x2. By selecting sixteen (S3=16) candidates of shorter square Euclidean distances from 256 patterns of combinations of signal points in this case, the candidates are narrowed down.
Similar processing is carried out by deciding section 72-1 for the fourth stage (here, the final stage). In this stage, the equation related to z1 is focused upon. Matrix elements r11, r12, r13 and r14 are known and combinations of transmission signals x2, x3 and x4 are narrowed down to sixteen patterns of candidates in the previous stage, and so there are sixteen signal point candidates of x1. Consequently, symbol candidate generating section 74-1 generates sixteen candidates related to x1. By selecting sixteen patterns of candidates (S4=16) of shorter square Euclidean distances from 256 patterns of combinations of signal points in this case, the candidates are narrowed down.
By limiting the number of candidates to equal to or less than a certain number (for example, S1≦C) in each stage in this way, signal point candidates of transmission signals can be narrowed down without calculating the square Euclidean distances for all possible combinations of signal points.    Patent Document 1: Japanese Patent Application Laid-Open No. 2006-157390